Because of its low octane rating and, therefore, low value as a motor fuel blending component, natural gasoline is advantageously fractionated into narrower boiling cuts, which have specialized applications and are often considerably more valuable than the natural gasoline feed. For instance, the pentane fraction or the separated fractions of n-pentane and iso-pentane, are useful as blowing agents in the preparation of foamed plastics. One requirement of blowing agents are that they be odor free and, therefore, desulfurization is required when the natural gasoline feed contains odoriferous sulfur compounds. Generally, such a desulfurization includes a caustic wash of either the natural gasoline feed or the pentane fraction(s) obtained by distillation of the gasoline. This caustic wash removes the mercaptans as is well known in the art. In the distillation to produce the narrow pentane fraction dimethyl sulfide remains with the pentane produced even after rectification because of the proximity of their respective boiling points (dimethylsulfide: 37.5.degree. C., n-pentane: 36.1.degree. C.). A convenient method for removing dimethylsulfide from the pentane product is by adsorption onto molecular sieves. In most cases, the combined caustic wash treatment and molecular sieve absorption will result in an odor-free pentane product containing usually less than 1 ppm of sulfur.
However, in some cases there still remains a problem with sulfur contamination of pentane fractions even after rigorous distillations, caustic washings and molecular sieve treatments which theoretically should be sufficient to remove any organic sulfur compounds boiling in the vicinity of the pentane boiling point.
It is therefore an object of this invention to provide a process for the substantially complete desulfurization of pentane.